| Glass Type | Susceptibility to Thermal Stress | Reason | |------------|----------------------------------|--------| | Annealed (float) glass | | Low tensile strength; no internal stress distribution | | Heat-strengthened glass | Medium | ~2× stronger than annealed, but can still crack under severe ΔT | | Tempered glass | Low | ~4–5× stronger; when it fails, it shatters into small pieces (not a single crack) | | Laminated glass | Medium (annealed core) | PVB interlayer holds fragments but does not prevent crack initiation |
A thermal stress crack is a type of glass fracture that occurs without physical impact, caused by rapid or extreme temperature differences across a window pane. Unlike impact damage (e.g., from a baseball or stone), thermal stress cracks typically begin at the glass edge and propagate inward. They are a common cause of window failure in both residential and commercial buildings, particularly in large fixed-pane windows, skylights, and insulated glass units (IGUs).
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Heavy drapes or furniture trapped against the glass.
Homeowners often confuse stress cracks with impact damage. However, stress cracks have distinct visual characteristics: What Causes a Thermal Stress Crack in Windows?
Thermal stress cracks occur when uneven heating causes different parts of a glass pane to expand at different rates. The Direct Cause
As the heated center tries to expand, the cool perimeter resists that movement.
Thermal stress cracks arise from real-world scenarios that create uneven heating or cooling:
| Feature | Thermal Stress Crack | Impact Crack | |---------|----------------------|----------------| | Origin | Edge or corner of pane | Point of impact (often center) | | Pattern | Single, straight or slightly curved line; perpendicular to glass edge | Concentric rings with radial lines (“spider web”) | | Surface | Smooth, no crater or bullseye | Crater, cone fracture, or bullseye mark | | Propagation | Runs from edge inward | Radiates outward from impact point |